| Rotary kiln is a critical equipment for cement production in which complex processes,e.g. material transportation, combustion, thermal exchange and chemical reaction occur simultaneously. The kiln has features like large volume, high energy consumption, long processing time, excellent fuel adaptability and etc. However, the rotary kiln is facing the shortage that its thermal efficiency is much lower than other thermal equipment. Therefore, the optimization of the mixing material and air, enhancing thermal exchange between phases and reducing heat loss is an urgent problem to be solved. To fully understand the mechanism of the multiphase flow and heat transfer in the kiln and, accordingly, improve its operating efficiency and reduce energy consumption, it is necessary to conduct deep investigation on the working processes in the kiln. Moreover, due to the complexity and typicalness of the problem, the research on kiln simulation has important theoretical significance to the discipline of dense gas-particle two-phase flow. In this thesis, taking a Φ4.3×64m cement rotary kiln as the engineering background, the complete kiln system, including a burner and a grate cooler, is numerically investigated based on multi-dimensional models.A burner with excellent performance could offer the best thermal condition for cement production. Hence, a research on a4-channel swirling burner is presented firstly, in which an Eulerian-Lagrangian approach is applied to simulate the turbulent field in the burner. Velocity distribution in the burner outlet area is analyzed at various inlet air flow conditions and various angles of the swirling blade, also, the characteristics of the turbulent jet flow field are studied under different cold conditions. The result shows that the central recirculation zone is important for a quick ignition and stable combustion; both the internal swirling flow and the high speed external flow contribute to an adequate mixing ofair and coal particles, and entrain the hot secondary air flow to enhance combustion. Increasing the high speed external flow can also promote the movement of the coal particles, however, it should be controlled in a reasonable range.Moreover, based on the Eulerian-Eulerian approach, a particle-air two-phase flow model is built for the rotary kiln to investigate the flow and heat transfer characteristics in a two-dimensional cross section and three-dimensional space, respectively. The kinetic theory for "particle fluid" is introduced in which an analogy between the random motion of particles and the thermal motion of molecules is adopted. A concept of granular temperature is used as a random parameter for describing particle kinetic energy, and the constitutive equation for the particle phase is established, which closes the solid momentum equation. A detailed discussion is presented for the heat exchange among the material, the wall and the air flow in free space where a heat source term is applied to calculating the temperature gradient in the axial direction. The mechanism of heat exchange between phases is discussed. Temperature distribution is obtained by numerical calculation to determine the heat quantity exchanged among phases, and between the material and the kiln wall. The result shows satisfactory agreement with experimental data, indicating that the model has a good prediction ability, and can provide useful guidance for the improvement and optimization of the operating process in the rotary kiln.Finally, focusing on the processes of the material transportation and heat exchange in the grate cooler and employing the Eulerian-Eulerian two-phase method and a porous medium model,2-D numerical simulation was carried out, to analyze heat exchange characteristics between various medium in the cooler. The calculation results show that the thickness of material bed increases significant with particle size increasing. Specifically, when the particle diameter reaches20mm, the best heat exchange efficiency and the highest temperature raise can be obtained. |
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